Injection molding machine for making a magnetic tape cassette

ABSTRACT

The present invention provides an injection molding die unit having a simple construction and an injection molding method employing the injection molding die unit and capable of molding a composite molding in a short cycle time. After molding a first molding (70), a male die (30) is moved together with the first molding (70) by a distance (Gb) in direction (L) and, at the same time, a movable female die (14B) of a composite female die (14) is moved by the same distance (Gb) in the same direction (L) by a coil spring (29) to form a space (72) between the male die 30 and the composite female die (14) consisting of the movable female die (14B) and a fixed female die (14A), and a second molding material is injected into the space (72) to form a second molding (80).

This is a division of application Ser. No. 07/885,454, filed May 19,1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an injection molding method to beapplied to forming a molding, such as a half case of a video tapecassette or an audio tape cassette, by sequentially injecting differentmolding materials into a die cavity formed between a male die and afemale die.

2. Description of the Prior Art

A half case of a video tape cassette or an audio tape cassette is formedby a two-color injection molding method of either a rotary system or acore-back system.

3. Rotary System

In forming a molding by a two-color injection molding method of a rotarysystem, a core die is turned in a plane perpendicular to a female die bya die inverting device to dispose the core die alternately opposite to afirst cavity die and a second cavity die for two-stage molding. Thetwo-color injection molding method of a rotary system needs two dies,i.e., the first cavity die and the second cavity die, needs a long cycletime because the die must be opened after the completion of molding inthe first cavity die to turn and transfer the first molding to thesecond cavity die and hence has a low productivity. Since the die isopened after the completion of the first molding cycle and a firstmolding is exposed to air before forming a composite molding bycombining a second molding formed in the second molding cycle, thecomposite molding is liable to separate into the first molding and thesecond molding along the interface between the first and secondmoldings.

4. Core-back System

A two-color injection molding method of a core-back system has beenproposed to solve the problems in the two-color injection molding methodof a rotary system. A typical two-color injection molding method of acore-back system is disclosed in Japanese Unexamined Patent Publication(Kokai) No. Sho 63-293024 (Method of Molding a Two-color Half Case of aTape Cassette). The gist of the invention disclosed in JapaneseUnexamined Patent Publication (Kokai) No. Sho 63-293024 will bedescribed hereinafter with reference to FIG. 18, which is a copy of FIG.1 included in the specification of the same invention.

In injecting a first material into a cavity formed in a die consistingof female die 110 and a male die 115, a slide core 126 for formingwindows in a half case of a tape cassette, movable on the male die 115is projected toward the female die 110, a cavity core 122 movable on thefemale die 110 is projected so as to close spaces for the windows andthe surface of a rim, and the first molding material is injected into acavity formed between the female die 110 and the male die 115. Then, theslide core 126 and the cavity core 122 are retracted to form spaces tobe filled up with a second molding material, and then the second moldingmaterial is injected into the spaces to complete a half case.

An invention disclosed in Japanese Unexamined Patent Publication (Kokai)No. Sho 57-187228 proposed to eliminate disadvantages in this two-colorinjection molding method of a core-back system that dimples are formedin the surface of the molding and the die is subject to damaging employsa slide core 5 provided with a recess 16 in its contact surface facing agate as shown in FIG. 19a and 19b, which is a copy of FIG. 4 included inthe specification of Japanese Unexamined Patent Publication (Kokai) No.Sho. 57-187228.

The die employed in the two-color injection molding method of acore-back system has a complicated construction, which affects adverselyto molding efficiency and causes the following problems.

1. Low productivity

Since the die has a complicated construction, it is difficult to form aneffective cooling passage in the die, the core-back system needs a longcycle time, troubles occur frequently during the process and thereliability of the die is not high enough.

2. Low economic effect

The die having a complicated construction is expensive and requires muchtime for fabrication. Nevertheless, it is difficult to obtain a diecapable of satisfactorily improving the productivity.

3. Low degree of freedom of design

There are many restrictions on the design of the die to give highpriority to reliability and it is difficult to incorporte improvementsinto the die.

The foregoing method of preventing defects attributable to a gate drophas the following problems.

1. Problems in commercial value

Since the depth of the recess formed in the contact surface of the slidecore facing the gate must be considerably large to avoid crushing thegate drop by the slide core, grooves having a pattern spoiling thedesign of the half case are formed around the transparent portion of thehalf case, for example, a portion around the windows.

2. Problems in strength

The wall thickness of the portion of the half case in which the groovesare formed in an undersirable pattern around the windows is smaller thanthat of other portions of the half case, which affects adversely to thestrength of the half case.

3. Problems in production

In some cases, the die was broken due to fatigue when the width of thecontact surface was increased to attach importance to design and thewide contact surface obstructed the flow of the molten resin todeteriorate moldability.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve those problemsin the prior art.

An injection molding method in one aspect of the present inventioncomprises steps of: injection-molding a first portion of a half case byinjecting a first molding material into a cavity defined by a male dieand a female die for a first molding cycle; cooling the first portion;separating the first portion together with either the male die or thefemale die from the other die; injection-molding a second portion of thehalf case by injecting a second molding material into the cavity definedby the male die and the female die for a second molding cycle; and, ifnecessary, repeating the second molding cycle to complete the half case.This injection molding method in accordance with the present inventionwill be designated as "body-back injection molding method" todiscriminate the injection molding method from the prior art injectionmolding method.

The body-back injection molding method in accordance with the presentinvention enables the fabrication of a multicolor-laminate molding or amulticolor-blocked molding by sequentially carrying out injectionmolding cycles in which molding materials of different colors are used.Thus, the present invention is applicable to fabricating moldings oforiginal designs, enables the use of an injection molding die having asimple construction and curtails the cost and time required forfabricating the injection molding die.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description takenin connection with the accompanying drawings, in which:

FIG. 1 is a sectional view of an injection molding die included in aninjection molding machine in a first embodiment according to the presentinvention for carrying out an injection molding method embodying thepresent invention;

FIG. 2 is a sectional view of assistance in explaining a step of theinjection molding method embodying the present invention;

FIG. 3 is a sectional view of assistance in explaining a step of theinjection molding method subsequent to the step explained with referenceto FIG. 2;

FIG. 4 is a sectional view of assistance in explaining a step of theinjection molding method subsequent to the step explained with referenceto FIG. 3;

FIG. 5 is a sectional view of assistance in explaining a step of theinjection molding method subsequent to the step explained with referenceto FIG. 4;

FIG. 6A is a plan view of a half case of an 8 mm video tape cassetteformed by the injection molding method embodying the present invention;

FIG. 6B is a sectional view taken on line A--A in FIG. 6A;

FIGS. 7A and 7B are sectional views of a hydraulic cylinder actuatorbuilt in a first modification of the injection molding die unit shown inFIG. 1;

FIGS. 8A and 8B are sectional views of a hydraulic cylinder actuatoremployed built in a second modification of the injection molding dieunit shown in FIG. 1;

FIG. 9 is a sectional view of an injection molding die unit included inan injection molding machine in a second embodiment according to thepresent invention;

FIG. 10 is a sectional view of assistance in explaining the operation ofthe injection molding machine in the second embodiment according to thepresent invention.

FIG. 11 is a sectional view of assistance in explaining the operation ofthe injection molding machine in the second embodiment according to thepresent invention;

FIG. 12A is a plan view of a half case of a compact tape cassette,formed by an injection molding method in a third embodiment according tothe present invention;

FIG. 12B is a sectional view taken on line A--A in FIG. 12A;

FIG. 13 is a sectional view of an injection molding die unit included inan injection molding machine in a third embodiment according to thepresent invention;

FIG. 14 is a sectional view of assistance in explaining the operation ofthe injection molding machine in the third embodiment according to thepresent invention;

FIG. 15 is a sectional view of assistance in explaining the operation ofthe injection molding machine in the third embodiment according to thepresent invention;

FIGS. 16A and 16B are enlarged sectional views of assistance inexplaining a gate drop, showing a portion in the vicinity of a secondgate in an injection molding die;

FIG. 17 is a plan view of a half case of a compact tape cassette, formedby an injection molding method in accordance with the present invention;

FIG. 18 is a fragmentary sectional view of assistance in explaining aprior art injection molding method; and

FIG. 19A and 19B are a fragmentary sectional view of assistanceexplaining measures for solving problems attributable to a gate drop incarrying out a prior art injection molding method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view of an injection molding machine for carryingout an injection molding method in a first embodiment according to thepresent invention, and FIGS. 2 to 5 show the injection molding machinein different steps of the injection molding method, in which FIG. 2shows a state where a first molding material has been injected into adie for the first injection molding cycle, FIG. 3 shows a state where afirst partial molding has been moved, FIG. 4 shows a state where asecond molding material has been injected into the die for the secondinjection molding cycle to form a second partial molding and FIG. 5shows a state where a complete molding consisting of the first andsecond partial moldings is being removed from the die. FIGS. 6A and 6Bshow a hale case of an 8 mm video tape cassette.

The injection molding method will be described as applied to forming ahalf case of an 8 mm video tape cassette.

Referring to FIGS. 6A and 6B, a half case 1 of an 8 mm video tapecassette consists of a bottom wall 4, and a side wall 3 extending alongthe three sides of the bottom wall 4, i.e., the sides other than thefront side 2 to which a front cover, not shown, which is opened inoperating a magnetic tape, not shown, contained in the video tapecassette and closed to protect the magnetic tape is connected. Thebottom wall 4 is provided with windows 5 to enable observation of themagnetic tape and has covering sections 6 and 7. The side wall 3 and thecovering section 7 are formed, for example, of an opaque colored moldingmaterial, and the windows are formed, for example, of a transparent ortranslucent colored molding material. The covering section 6 has atwo-layer laminate construction consisting of a lower layer formed ofthe molding material forming the covering section 7 integrally with thecovering section 7, and an upper layer formed of the molding materialforming the windows 5 integrally with the windows 5. Note that a moldingof an original, beautiful design, which has been difficult to fabricate,can be relatively easily fabricated at a relatively low cost by moldingmolding materials differing from each other in transparency, colorand/or quality.

The construction of an injection molding machine suitable for moldingthe half case 1 will be described hereinafter with reference to FIG. 1showing an injection molding die unit 10 consisting of a fixed dieassembly 10A and a movable die assembly 10B, and the associated parts.

The fixed die assembly 10A has a composite female die 14 consisting of afixed female die 14A and a movable female die 14B. The fixed dieassembly 10A is held fixedly on the datum surface 12 of a base 11 by aholding plate 13. A cavity surface corresponding to the outer surface ofthe half case 1 is formed in the composite female die 14. A firstinjector 15 having a first injection nozzle 16 is held on the base 11and the first injection nozzle 16 extends through a manifold block 21,the holding plate 13 and the movable female die 14B. The gate 17 of thefirst injection nozzle 16 opens in the cavity surface of the movablefemale die 14B. A second injector 18 held on the base 11 has a secondinjection nozzle 19 extending through the manifold block 21 and thefixed female die 14A. The gate 20 of the second injection nozzle 19opens in the cavity surface of the fixed female die 14A. The movablefemale die 14B is provided with a guide hole 22, and a guide rod 23projecting from the holding plate 13 and perpendicular to the surface ofthe holding plate 13 is received in the guide hole 22 to guide themovable female die 14B for axial movement. Recesses 25 and 26 separatedby a partition wall 24 provided with a through hole are formed in themovable female die 14B. A headed rod 27 having a head and a straightsection is inserted through the through hole in the recesses 25 and 26with the head thereof received in the recess 25, with the straightsection thereof extending in the recess 26 and with the extremity 28 ofthe straight section in contact with the surface of the holding plate13. A coil spring 29 is put in the recess 26 around the straight sectionof the headed rod 27 to bias the movable female die 14B constantly tothe left, as viewed in FIG. 1, i.e., toward the movable die assembly10B, so that a gap is formed between the fixed female die 14A and themovable female die 14B. The coil spring 29 may be substituted by ahydraulic unit or the like.

The movable die assembly 10B of the injection molding die unit 10 has amale die 30 having a core surface corresponding to the inner surface ofthe half case 1 and provided with a guide hole 31. A guide rod 33projecting from a base plate 32 and perpendicular to the surface of thebase plate 32 is received in the guide hole 31 of the male die 30 toguide the male die 30 for axial movement. A hydraulic cylinder actuator50 is built in the male die 30 to move the male die 30 toward thecomposite female die 14.

The hydraulic cylinder actuator 50 has a pressure chamber 51 formed inthe left end of the male die 30, a piston 52 fitted in the pressurechamber 51, a piston ring 53 put on the circumference of the piston 52,and a shaft 54 having one end fixed to the piston 52 and the other endfixed to the base plate 32. FIGS. 7A, 7B, 8A and 8B show modificationsof the hydraulic cylinder actuator 50, which will be described later.

The male die 30 is provided with two parallel guide holes 34 and 35parallel to the guide hole 31. The guide holes 34 and 35 open into achamber 36 having a rectangular cross section. An ejecting pin 37 and areturn pin 38 projecting from the surface of an ejecting plate 39disposed within the chamber 36 and perpendicular to the same surface arereceived respectively in the guide holes 34 and 35. The ejecting pin 37and the ejecting plate 39 separate a molding from the male die 30 andeject the same from the injection molding die unit 10. The chamber 36has a width large enough to enable the ejecting plate 39 to be pushed byan ejecting rod to the right, as viewed in FIG. 1, to eject a molding. Athrough hole 41 is formed in the back portion of the male die 30 toreceive the ejecting rod 40 therethrough into the chamber 36. Althoughonly one ejecting pin 37 is shown in FIG. 1, the male die 30 is providedwith a plurality of ejecting pins; the number of the ejecting pins isdependent on the size and shape of a molding to be molded in theinjection molding die unit 10. The male die 30 is attached to the baseplate 32 which in turn is fixed to the die base 42 of the injectionmolding machine.

Steps of forming the half case shown in FIGS. 6A and 6B by the injectionmolding method employing the injection molding die unit 10 thusconstructed will be described hereinafter with reference to FIGS. 2 to5.

FIG. 2 shows the injection molding die unit 10 in a state for the firstinjection molding cycle for forming a first molding 70. The injectionmolding die unit 10 is mounted on the injection molding machine with thefixed die assembly 10A fixed to the injection molding machine and withthe movable die assembly 10B fixed to the die base 42 of the hydraulicunit, not shown. A predetermined pressure is applied to the die base 42in the direction of the arrow X to close the injection molding die unit10. A hydraulic pressure is applied to a front chamber Pa of thepressure chamber 51, a pressure chamber in front of the piston 52, ofthe hydraulic cylinder actuator 50 to move the male die 30 in thedirection of the arrow R. Then, the upper and lower recessed surfaces ofthe male die 30 comes into contact respectively with the upper and lowerprotruding surfaces of the movable female die 14B of the compositefemale die 14 and pushes the movable female die 14B backward against theresilience of the coil spring 29 to press the back surface of themovable female die 14B against the holding plate 13. In this state, thefront end of the guide rod 23 is received in the guide hole 31 and incontact with the guide rod 33 to define a gap of a size Ga between thecavity surface of the composite female die 14 and the core surface ofthe male die 30 defining a space 71 into which a first molding materialis injected. The size Ga of the gap is equal to the thickness Ga of thecovering section 6 of the bottom wall 4 of the half case 1 (FIG. 6B). Adistance Gb by which the movable female die 14B is pushed backward isequal to the width of a space into which a second molding material isinjected, i.e., the thickness of a second layer. The space 71 consistsof a space 71a corresponding to the covering sections 6 and 7 of thehalf case 1, and a space 71b corresponding to the rim 3 of the half case1 (FIGS. 6A and 6B).

A first injecting unit 15 injects the first molding material, forexample, a colored plastic resin, such as a black plastic resin, throughthe manifold block 21 and the first injection nozzle 16 into the space71 having a width equal to the size Ga, and then the first moldingmaterial injected into the space 71 is cooled to form the first molding70 forming a first layer of a first color of the half case 1.

Subsequently, hydraulic pressure is removed from the front chamber Paand applied to a back chamber Pb, i.e., a pressure cheer behind thepiston 52, of the hydraulic cylinder actuator 50. Then, as shown in FIG.3, the male die 30 of the movable die assembly 10B is moved back wardtogether with the first molding 70 in the direction of the arrow La by adistance equal to the thickness of a second layer to be formed, i.e., adistance equal to the distance Gb and, consequently, the back surface ofthe male die 30 is brought into contact with the surface of the baseplate 32. At the same time, the movable female die 14B is moved in thedirection of the arrow Lb by the resilience of the coil spring 29. Whilethe movable female die 14B is being moved in the direction of the arrowLb, the upper and lower protruding surfaces of the movable female die14B remain in contact respectively with the upper and lower recessedsurfaces of the make die 30. Consequently, the cavity surface of thecomposite female die 14 as shown in FIG. 1 is formed, and a space 72 isformed between the cavity surface of the composite female die 14 and theexposed surface, i.e., the surface facing the cavity surface of thecomposite female die 14, of the first molding 70 adhering to the coresurface of the male die 30. The space 72 corresponds to a secondmolding. The space 72 has a portion 72a corresponding to the windows 5of the half case 1 (FIG. 6A) and a portion 72b corresponding to an upperlayer for the covering section 6 of the half case 1.

Then, as shown in FIG. 4, a second molding material, for example, atransparent second plastic resin of a second color, is injected throughthe manifold block 21, the second injection nozzle 19 and the gate 20into the space 72 by the second injector 18 for the second injectionmolding cycle to form a second molding 80 forming a second layer, andthen, the second plastic material injected into the space 72 is cooled.Thus, the transparent windows 5 of a single-layer construction and thecovering sections 6 and 7 of a two-layer construction consisting of thefirst layer of the black first molding formed by the first injectionmolding cycle and the second layer of the transparent second moldingformed by the second injection molding cycle.

Subsequently, as shown in FIG. 5, the movable die assembly 10B in thedirection of the arrow Y by the hydraulic unit to open the injectionmolding die unit 10 and the ejecting plate 39 is pressed by the ejectingrod 40 to separate the half case 1 of a two-layer laminate constructionfrom the male die 30 and to eject the half case 1 from the injectionmolding die unit 10 in the direction of the arrow Z with the ejectingpin 37.

As is obvious from the foregoing description, the injection molding dieunit 10 is constructed so that the male die 30 with the first molding 70and the movable female die 14B can be moved toward the die base 42 bythe distance Gb equal to the width of the space 72 for molding thesecond molding, and the distance Gb is determined by the length of theheaded rod 27. Thus, this injection molding method is designated as"body-back injection molding method".

It is possible, on principle, to interchange the fixed die assembly 10Aand the movable die assembly 10B, namely, it is possible to mount thefixed die assembly 10A including the composite female die 14 on amovable member of the injection molding machine and to mount the movabledie assembly 10B including the male die 30 on the fixed member of theinjection molding machine for the same effect.

The hydraulic cylinder actuator 50 may be built in the base plate 32 orthe die base 42 of the injection molding machine instead of building thesame in the male die 30. FIGS. 7A and 7B show a hydraulic cylinderactuator built in a base plate and employed in an injection moldingmachine in a second embodiment according to the present invention andFIGS. 8A and 8B show a hydraulic cylinder actuator built in a die baseand employed in an injection molding machine in a third embodimentaccording to the present invention, in which parts like or correspondingto those of the injection molding machine shown in FIG. 1 are denoted bythe same reference characters.

The injection molding machine in the second embodiment according to thepresent invention will be described with reference to FIG. 7Acorresponding to FIG. 2, and FIG. 7B corresponding to FIG. 3. Ahydraulic cylinder actuator 50 has a pressure chamber 51 formed in abase plate 32, a piston 52 fitted in the pressure chamber 51, a pistonring put on the circumference of the piston 52, and a shaft 54 havingone end connected to the piston 52 and the other end fixed to a male die30. A hydraulic pressure is applied to a back chamber Pa, i.e., achamber behind the piston 52, as shown in FIG. 7A to move the male die30 and the piston 52 in the direction of the arrow R with the shaft 54so that a gap of a width Gb is formed between the male die 30 and thebase plate 32. The width Gb is equal to the width of the space intowhich the second molding material is injected to form the second moldingforming the second layer. Hydraulic pressure is removed from the backcheer Pa and applied to a front chamber Pb, i.e., a chamber in front ofthe piston 52, as shown in FIG. 7B to pull the male die 30 with theshaft 54 so that the male die 30 is brought into contact with the baseplate 32.

The injection molding machine in a third embodiment according to thepresent invention will be described with reference to FIGS. 8A and 8B.The injection molding machine is provided with a hydraulic cylinderactuator 50 built in its die base 42. The hydraulic cylinder actuator 50has a pressure cheer 51 formed in the die base 42, a piston 52 fitted inthe pressure chamber 51, a piston ring 53 put on the circumference ofthe piston 52, and a shaft 54 having one end fixed to the piston 52 andthe other end extending through a through hole 55 formed in a base plate32 and joined to the back surface of a male die 30. The shaft 54 isfastened to the male die 30 with a screw 56. The operation of thehydraulic cylinder actuator 50 for forming the gap of the width Gb isthe same as that of the hydraulic cylinder actuator 50 shown in FIGS. 7Aand 7B, and hence the description thereof will be omitted.

Although the injection molding method embodying the present inventionhas been described as applied to molding the half case 1 of a two-color,two-layer laminate construction, it is possible to apply the injectionmolding method to fabricating a molding of a multi-color, multilayerlaminate construction by employing an appropriate modification of thecomposite female die 14.

An injection molding machine in a second embodiment according to thepresent invention will be described with reference to FIGS. 9 to 12B asapplied to molding a half case of a compact tape cassette. FIG. 9 is asectional view showing an essential portion of the injection moldingmachine, FIGS. 10 and 11 show the injection molding machine in the firstinjection molding cycle and in the second injection molding cycle,respectively, and FIGS. 12A and 12B show a half case of a compact tapecassette formed by the injection molding method, in which parts like orcorresponding to those of the injection molding machine previouslydescribed with reference to FIGS. 1 to 6B are denoted by the samereference characters.

Referring to FIGS. 12A and 12B, a half case 1A, i.e., a front half caseof a compact tape cassette, has a bottom wall 4A, a side wall 2Aextending along one side edge of the bottom wall 4A so as to expose amagnetic tape, not shown, contained in the compact tape cassette, and arim 3A extending along the three side edges of the bottom wall 4A. Thebottom wall 4A is provided with a transparent window 5A to enableobservation of the magnetic tape contained in the compact tape cassette,a transparent labeling section 8 determining a labeling area andenabling observation of the internal mechanism of the compact tapecassette, a decorative transparent section 9A allowing observation of apad, decorative transparent sections 9B allowing observation of guiderollers, and covering sections 6A and 7A. For example, the rim 3A andthe covering section 7A are formed of an opaque molding material of acolor in a single-layer construction, the window 5A is formed of atransparent or translucent molding material of a color in a single-layerconstruction, the covering section 6A is formed in a two-layer laminateconstruction consisting of a lower layer formed of the same material asthat forming the covering section 7A integrally with the coveringsection 7A, and an upper layer formed of the same material as thatforming the window 5A integrally with the window 5A.

The injection molding machine in the second embodiment according to thepresent invention employs an injection molding die unit 200 shown inFIG. 9 consisting of a fixed die assembly 200A and a movable dieassembly 200B.

The fixed die assembly 200A shown in FIG. 9 is substantially the same inconstruction as the fixed die assembly 10A shown in FIG. 1, except thatthe fixed die assembly 200A is not provided with any rod correspondingto the rod 23 of the fixed die assembly 10A shown in FIG. 1 and asupport rod 233 included in the movable die assembly 200B is substitutedfor the rod 23. Therefore, parts shown in FIG. 9 like or correspondingto those shown in FIG. 1 are denoted by the same reference charactersand the description thereof will be omitted to avoid duplication.Although a single support rod 233 is shown in FIG. 9 for the sake ofconvenience, it is desirable to provide the movable die assembly 200Bwith a plurality of support rods in an appropriate arrangement so thatthe support rods may not obstruct the ejection of a molding.

The movable die assembly 200B has a composite male die 30 consisting ofa movable male die 30A and a movable male die 30B. Most part of themovable male die 30B lies between a base plate 32 and the movable maledie 30A. A protruding part of the movable male die 30B protrudes to theright through the movable male die 30A. The right end surface 202 of themovable male die 30A and the right end surface 203 of the protrudingpart of the movable male die 30B form a cavity surface corresponding tothe window 5 of the half case 1A. This cavity surface and the right endsurfaces 201 and 202 of the movable male die 30A form a cavity surfacecorresponding to the inner surface of the half case 1A. The movable maledies 30A and 30B are provided respectively with guide holes 31A and 31B,and the support rod 233 attached perpendicularly to the base plate 32 isreceived in the guide holes 31A and 31B to guide the movable male die30B for movement toward the movable male die 30a and further toward afemale die 14 included in the fixed die assembly 200A. A hydrauliccylinder actuator 50 built in the movable male die 30B moves the movablemale die 30B along the support rod 233. The support rod 233 guides alsothe movable male die 30A for movement toward the male die 14 of thefixed die assembly 200A and for slight movement toward the movable maledie 30B. The distance of movement of the movable male die 30A toward themovable male die 30B is determined by a positioning rod 234. When thefixed die assembly 200A and the movable die assembly 200B are joinedtogether, the front portion of the support rod 233 is received in aguide hole 22 formed in a movable female die 14B included in the fixeddie assembly 200A to guide the movable female die 14B for movementtoward the movable die assembly 200B. It is desirable to provide themovable die assembly 200B with as many support rods as possible to applyclamping force uniformly to the injection molding die unit 200. Themovable male dies 30A and 30B are provided respectively with recesses235 and 236. A headed rod 237 is put on the movable die assembly 200Bwith its head received in the recess 235 and its straight sectionreceived in the recess 236. A coil spring 238 is placed in the recess236 around the straight section of the headed rod 237 to urge themovable male die 30A constantly toward the fixed die assembly 200A.

The hydraulic cylinder actuator 50 has a pressure chamber 51 formed inthe left end of the movable male die 30B, a piston 52 fitted in thepressure chamber 51, a piston ring 53 put on the circumference of thepiston 52, and a shaft 54 having one end fixed to the piston 52 and theother end fixed to the base plate 32. Naturally, the hydraulic cylinderactuator 50 may be substituted by the hydraulic cylinder actuator shownin FIGS. 7A and 7B or in FIGS. 8A and 8B.

Naturally, the injection molding machine in the second embodiment needsa mechanism similar to the mechanism comprising the ejecting pin 37, thereturn pin 38, the ejecting plate 39 and the ejecting rod 40, includedin the injection molding machine in the first embodiment shown in FIG.1, for removing a molding from the male die 30. The description of themechanism will be omitted for the sake of simplicity.

An injection molding method employing the injection molding die unit 200thus constructed will be described hereinafter with reference to FIGS.10 and 11 as applied to molding the half case 1A shown in FIGS. 12A and12B.

The injection molding die unit 200 shown in FIG. 10 is in a state formolding a first molding 70A. The injection molding die unit 200 ismounted on the injection molding machine with the fixed die assembly200A fixed to a fixed member of the injection molding machine and themovable die assembly 200B fixed to a die base, not shown, connected to ahydraulic device. The movable die assembly 200B is pressed in thedirection of the arrow X by a predetermined pressure to close theinjection molding die unit 200.

Hydraulic pressure is applied to a front chamber Pa of the hydrauliccylinder actuator 50 to move the movable male die 30B together with themovable male die 30A in the direction of the arrow R by a distancecorresponding to the thickness Ta of the walls of the half case 1A.Consequently, the upper and lower recessed surfaces of the movable maledie 30A come into contact with the upper and lower protruding surfacesof the movable male die 14B of the composite female die 14 and press themovable female die 14B against the resilience of the coil spring 29 topress the back surface of the movable female die 14B against the holdingplate 13. The front end of the support rod 233 advances into the guidehole 22 and comes into contact with the holding plate 13 to limit theadvancement of the movable male die 30A and the movable male die 30B.The distance between the end surface of the movable male die 30B and thecorresponding end surface of the movable female die 14B is equal to thethickness Ta of the covering section 7A and the first layer of the rim3A (FIG. 12B), is equal to the thickness Ta of the bottom wall. Thedistance between the cavity surface of the movable male die 30B and thecavity surface of the fixed male die 14A, i.e., the thickness of thefirst layer of the covering section 6A (FIG. 12B) is Ta-Tb, where Tb isthe thickness of the second layer 80A (FIG. 11). A first moldingmaterial, such as a plastic resin of a first color, for example, black,is injected in the spaces through the gate 17 of a first injectiondevice 15 to form a first molding of the first color forming a firstlayer, and then the first molding is cooled.

Then, hydraulic pressure is applied to a back chamber Pb of thehydraulic cylinder actuator 50 to move the male die 30 of the movabledie assembly 200B together with the first molding in the direction ofthe arrow La as shown in FIG. 11. When the male die 30 is thus moved,the movable male die 30B moves by a distance equal to the thickness Tband the movement of the movable male die 30A is limited by thepositioning rod 234 to form a gap of a width equal to the differencebetween the thickness Ta and Tb, i.e., Ta-Tb, between the back surfaceof the movable male die 30a and the front surface of the movable maledie 30B. In this state, the back surface of the movable male die 30B isin contact with the surface of the base plate 32. At the same time, themovable female die 14B is moved in the direction of the arrow Lb by theresilience of the coil spring 29 to keep the upper and lower protrudingsurfaces of the movable female die 14B in contact with the correspondingupper and lower recessed surfaces of the male die 30. Thus, the cavitysurface of the movable female die 14B becomes flush with the cavitysurface of the fixed female die 14A (the state of the cavity surface ofthe female die 14 shown in FIG. 9) and a space of the width Tb is formedbetween the cavity surface of the female die 14 and the free surface ofthe first molding 70A closely adhering to the core surface of the maledie 30. A second molding is formed in this space and a space defined bythe cavity surface of the movable male die 30A and the cavity surface ofthe fixed female die 14A. Portions of these spaces correspondrespectively to the window 5A, a labeling section 8, the transparentsection 9A and the upper layer of the covering section 6A.

A second molding material, for example, a transparent plastic resin of asecond color, is injected for the second injection molding cycle intothose spaces through the gate 20 of a second injection device to form asecond molding 80A of a second color forming the second layer, and thenthe second molding 80A is cooled. In the second injection molding cycle,the window 5A, the labeling section 8 and the transparent section 9A areformed in a single-layer construction. Other sections including thecovering sections 6A and 6B are formed in a two-layer construction andthese sections appear through the transparent second molding 80A in thecolor of the first molding, i.e., black, formed by the first injectionmolding cycle.

Then, the hydraulic device is operated to separate the movable dieassembly 200B from the fixed die assembly 200A and then the half case 1Aof a two-layer construction is removed from the male die 30 and ejectedfrom the injection molding die unit 200 by the ejecting mechanism.

As is obvious from the foregoing description, since the movable dieassembly is provided with the built-in hydraulic cylinder actuator andthe positioning mechanism, the cavity-back mechanism and the core-backmechanism essential to the prior art can be omitted, and the injectionmolding die unit 200 has a very simple construction.

An injection molding machine in a third embodiment according to thepresent invention suitable for carrying cut an injection molding methodof a body-back system in accordance with the present invention capableof preventing dimples attributable to a gate drop in transparentsections, such as the window of a half case of a tape cassette will bedescribed with reference to FIGS. 13 to 17.

FIG. 13 is a sectional view of of a molding die unit employed in theinjection molding machine in the third embodiment according to thepresent invention, FIG. 14 shows the molding die unit of FIG. 13 in astate after the completion of the first injection molding cycle forforming a first molding, FIG. 15 shows the molding die unit in a stateafter the completion of the second injection molding cycle for forming asecond molding, FIGS. 16A and 16B are enlarged, fragmentary sectionalviews of assistance in explaining a gate drop and FIG. 17 shows a halfcase of a compact tape cassette molded by the injection molding methodof a back-body molding system, in which parts like or corresponding tothose of the molding die units 10 and 200 shown in FIGS. 1 to 12B aredenoted by the same reference characters.

First a half case 1B of a compact tape cassette molded by the injectionmolding method of the present invention will be described with referenceto FIG. 17. The half case 1B has a bottom wall 4B, a side wall 2Bextending along one side edge of the bottom wall 4B and having anopening through which a magnetic tape, not shown, contained in thecompact tape cassette is exposed, and a rim 3B extending along the otherthree sides of the bottom wall 4B. The bottom wall 4B has a transparentor translucent window 5B to enable observation of the magnetic tapecontained in the compact tape cassette, and an opaque covering section6B, i.e., a hatched area in FIG. 17. The rim 3B and the covering section6B are formed of an opaque molding material of a color in a single-layerconstruction, and the window 5B is formed of a transparent moldingmaterial of a color in a single-layer construction. All the portions ofthe half case 1B are the same in thickness.

Referring to FIG. 13, an injection molding die unit 300 employed in theinjection molding machine in the third embodiment consists of a fixeddie assembly 300A and a movable die assembly 300B.

The fixed die assembly 300A is the same in construction as the fixed dieassembly 200A shown in FIG. 9 and hence the description thereof will beomitted to avoid duplication.

The movable die assembly 300B has a male die 30 consisting of a movablemale die 30C and a fixed male die 30D penetrating the central portion ofthe movable male die 30C. The right end surfaces 301 and 302 of themovable male die 30C and the right end surface 303 form a core surfacecorresponding to the inner surface of the half case 1B. The right endsurfaces 302 and 303 form a core surface corresponding to thetransparent or translucent window 5B. In FIG. 13, the movable male die30C is moved toward the female die 14 of the fixed die assembly 300A.The movement of the movable male die 30C toward the female die 14 islimited by the base plate 32 so that the right end surface 302 of themovable male die 30C projects from the right end surface 303 of thefixed male die 30D by a distance D. The distance D is large enough toform a space large enough to contain a gate drop 310 as shown in FIG.16A and corresponds to the thickness of a second molding 80B, which isequal to the thickness of a first molding 70B, i.e., the thickness ofthe walls of the half case 1B.

The movable male die 30C is provided with a guide hole 31, and a supportrod 233 attached perpendicularly to the base plate 32 is received in theguide hole 31 to guide the movable male die 30C when the movable maledie 30C is moved toward the female die 14 of the fixed die assembly 300Aby a hydraulic cylinder actuator 50 built in the movable male die 30C.When the fixed die assembly 300A and the movable die assembly 300B arejoined together, the front end of the support rod 233 advances into aguide hole 22 formed in the movable female die 14B of the fixed dieassembly 300A to guide the movable female die 14B for movement towardthe movable die assembly 300B. As mentioned in the description of thesecond embodiment, it is desirable to provide as many support rods aspossible to apply clamping force uniformly to the injection molding dieunit 300 in an arrangement that may not obstruct the ejection of amolding.

The hydraulic cylinder actuator 50 built in the left end portion of themovable male die 30C is the same in construction as those described withreference to FIGS. 1 and 9 and hence the description thereof will beomitted.

The injection molding machine in the third embodiment is provided withan ejecting mechanism similar to that employed in the first embodiment,comprising the ejector pin 37, the return pin 38, the ejecting plate 39and the ejecting rod 40, which is not shown in FIGS. 13 to 15 for thesake of simplicity.

Steps of the injection molding method employing the injection moldingdie unit 300 thus constructed for molding the half case 1B shown in FIG.17 will be described hereinafter with reference to FIGS. 14 and 15.

In FIG. 14, the injection molding die unit 300 is in a state for moldinga first molding 70B. The injection molding die unit 300 is mounted,similarly to those employed in the first and second embodiments, on theinjection molding machine and a predetermined pressure is applied to themovable die assembly 300B of the injection molding die unit 300 in thedirection of the arrow X for die clamping.

After the injection molding die unit 300 has been closed, hydraulicpressure is applied to a front chamber Pa of the built-in hydrauliccylinder actuator 50 to move the movable male die 30C together with themovable made die 30A in the direction of the arrow R by a distance equalto the wall thickness D of the half case 1B. The upper and lowerrecessed surfaces 301 of the movable male die 30C come into contact withthe corresponding upper and lower protruding surfaces of the movablefemale die 14B of the female die 14 to move the movable female die 14Bagainst the resilience of the coil spring 29 so that the back surface ofthe movable female die 14B is pressed against the holding plate 13. Thefront end of the support rod 233 advances through the guide hole 22 andcomes into contact with the holding plate 13 to limit the advancement ofthe movable male dies 30A and 30C. In this state, the central coresurface 302 of the movable male die 30C is in contact with the cavitysurface 304 of the fixed female die 14A. Thus, the space 305 shown inFIG. 16A is formed between the fixed female die 14A and the fixed maledie 30D facing the gate 20 of the fixed female die 14A.

Thus, a space of a width equal to the thickness of the covering section6B (FIG. 17), i.e., a space for forming the first molding 70B (FIG.16A), is defined by the core surface of the movable male die 30C and thecavity surfaces of the fixed female die 14A and the movable female die14B. A first molding material, for example, a plastic resin of a firstcolor, such as black, is injected into the space for forming the firstmolding 70B through the gate 17 of the first injection device 15 to moldthe first molding 70B, and then the first molding 70B is cooled.

In the state shown in FIG. 14, hydraulic pressure is removed from thefront chamber Pa and applied to a back chamber Pb. Then, as shown inFIG. 15, the movable male die 30C of the movable die assembly 300B movestogether with the first molding 70B in the direction of the arrow L by adistance equal to the wall thickness D. In this state, the back surfaceof the movable male die 30C is in close contact with the base plate 32and the core surface 302 of the movable male die 30C is flush with thecore surface 303 of the fixed male die 30D. At the same time, themovable female die 14B is moved in the direction of the arrow L by thecoil spring 29 as the movable male die 30C moves in the direction of thearrow L. The upper and lower protruding surface of the movable femaledie 14B remain in contact with the corresponding upper and lowerrecessed surfaces of the male die 30 as the movable female die 14B ismoved in the direction of the arrow L. Consequently, the cavity surfaceof the movable female die 14B becomes flush with the cavity surface ofthe fixed female die 14A to form the cavity surface of the female die 14as shown in FIG. 13 and a space 306 of a width equal to the wallthickness D (FIG. 16B) is formed between the cavity surface of thefemale die 14 and the core surface of the male die 30. The space 306corresponds to the window 5B of the half case 1B of FIG. 17.

Then, as shown in FIG. 15, a second molding material, for example, atransparent plastic resin of a second color, is injected into the space306 to form a second molding BOB, i.e., the window 5B. Thus, the halfcase 1B integrally consisting of the covering section 6B and the window5B is completed. If there is a gate drop 310 in the gate 20 in injectingthe second molding material into the space 306, the gate drop 310 ismelted by the heat of the second molding material and disperses in thesecond molding material, so that any grooves of a pattern spoiling thedesign of the half case 1B, which is the disadvantage of the prior art,are not formed around the window 5B.

Then, hydraulic device is operated to move the movable die assembly 300Bin the direction of the arrow Y to open the injection molding die unit300, and the half case 1B is removed from the male die 30 and ejectedfrom the injection molding die unit 300.

It is essential that the area of the core surface 303 of the fixed maledie 30D is greater at least than the area of the tip of the gate 20. Itwas found through experiments that the half case can be satisfactorilymolded when the gate 20 has a circular tip of 3 mm in diameter and thecore surface 303 has a circular shape of 4 mm in diameter. In FIG. 17, acircle 20B corresponds to the circular tip of the gate 20 and a circle303B corresponds to the circular core surface 303 of the fixed male die30D. The size and shape of the core surface 303 may be selectivelydetermined taking into consideration the size and shape of a molding tobe formed. If necessary, the core surface 303 of the fixed male die 30Dmay be as large as an area 307 enclosed by dotted lines in FIG. 17,which is nearly equal to the entire area of the window 5B.

As is obvious from the foregoing description of the third embodiment,since the movable die assembly is provided with the built-in hydrauliccylinder actuator and the positioning mechanism, the cavity-backmechanism and the core-back mechanism, which are essential to the priorart, may be omitted, which simplifies the construction of the moldingdie unit greatly. Since the fixed male die 30D is fixedly connected tothe base plate 32, the fixed male die 30D can be easily cooled.

Furthermore, even if there is a gate drop 310 in the gate 20 as shown inFIG. 16A in injecting the second molding material into the molding dieunit, a portion of the end surface of the molding die facing the gate 20is not damaged because the fixed male die 30D of the movable dieassembly 300B is formed so that the space 305 is formed between the endsurface of the fixed male die 30D and the gate 20. Still further, sinceany grooves of a pattern spoiling the design of the half case are notformed around the window, a half case of high aesthetic quality can bemolded in the molding die unit. Since any portion having a reducedthickness is not formed around the window, problems in the strength ofthe half case is solved.

Although the invention has been described in its preferred forms with acertain degree of particularity, obviously many changes and variationsare possible therein. It is to be understood that the present inventionmay be practiced otherwise than as specifically described herein withoutdeparting the spirit and scope thereof.

What is claimed is:
 1. An injection molding machine comprising: amolding die unit consisting of a male molding die assembly and a femalemolding die assembly mating with the male molding die assembly; and amovable die base; wherein either the male molding die assembly or thefemale molding die assembly is a movable molding die assembly and theother is a fixed molding die assembly, the movable molding die assemblyis attached to the movable die base, the fixed molding die assembly isattached to a fixed member of the injection molding machine, and themovable molding die assembly can be moved toward the fixed molding dieassembly by a moving means built therein or by a moving means formed onthe movable die base, wherein said female molding die assembly comprisesa first die component fixed to a female die base of the female dieassembly and a movable die component longitudinally slidable withrespect to said female die base and a longitudinal restraint extendingfrom said female die base and restricting longitudinal separation ofsaid movable component from said female die base.
 2. An injectionmolding machine comprising:a male molding die assembly; a female moldingdie assembly aligned to move into mating engagement with the malemolding die assembly along a longitudinal axis to define a moldingcavity therebetween; said female die assembly including a fixed femaledie and a movable female die, said fixed female die mounted onto a firstbase, and said movable female die mounted on said first base slidablywith respect to said first base and said fixed female die along saidlongitudinal axis and spring biased from said first base toward saidmale molding die assembly, said movable female die having a surroundingsurface abuttable to a compatible surrounding surface on said male dieassembly and retractable toward said first base by an urging of saidmale die assembly toward said female die assembly; and a first injectionnozzle through said fixed female die from outside said molding cavityand a second injection nozzle through said moveable female die fromoutside said molding cavity.
 3. The injection molding machine accordingto claim 2, wherein said movable female die surrounds said fixed femaledie.
 4. The injection molding machine according to claim 2, wherein saidmale die assembly is carried on a movable second base, said movablesecond die base translatable along said longitudinal axis, and saidfirst base fixed with respect to said injection molding machine.
 5. Theinjection molding machine according to claim 4, wherein said male dieassembly comprises a cylinder adjacent said movable second die base andsaid movable second die base comprises a piston fixed thereto andresiding for reciprocal movement within said cylinder, said cylindersized to allow a freedom of movement along said longitudinal axisbetween said movable second die base and said male die assembly, saidcylinder having means to receive fluid pressure to move said male dieassembly along said longitudinal axis with respect to said piston. 6.The injection molding machine according to claim 2, wherein said movablefemale die is guided by means of at least one pin, said pin fixed tosaid first base and projecting along the longitudinal axis and residingin a bore located in said movable female die.
 7. The molding machineaccording to claim 6, wherein said one pin provides a head portionlocated on a side of said movable female die opposite to said firstbase, said head portion providing a limit stop of movement of saidmovable female die from said base.
 8. The molding machine according toclaim 2 further comprising an ejecting pin mounted reciprocally withinsaid make die assembly and protrudable into said molding cavity to ejectsaid molded article from said male molding assembly.
 9. The moldingmachine according to claim 8, further comprising a return pin connectedto said ejecting pin, said return pin located to be engaged by saidfemale die assembly when said male die assembly is engaged to saidfemale die assembly to retract said ejecting pin from said moldingcavity.
 10. The molding machine according to claim 2, wherein saidmovable female die is translatable a discrete distance toward said firstbase before being stopped by said first base.